<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        header {
            width: 100%;
            height: 50px;
            background-color: lightblue;
            text-align: center;
            line-height: 50px;
        }

        section {
            display: flex;
        }

        aside {
            flex: 1;
            height: 600px;
            background-color: lightcoral;
            text-align: center;
            line-height: 600px;
        }

        main {
            flex: 8;
            height: 600px;
            background-color: lightpink;
            text-align: center;
            line-height: 600px;
        }
    </style>
</head>

<body>
    <div>
        <header>
            header
        </header>
        <section>
            <aside>
                aside
            </aside>
            <main>
                main
            </main>
        </section>
    </div>

</body>

</html>